Olefin polymerization systems are divided into Ziegler-Natta and metallocene catalyst systems, and these two highly active catalyst systems have been developed corresponding to particular characteristics. The Ziegler-Natta catalyst has been widely applied in existing commercial processes since it was invented in the fifties, but since it is a multi-site catalyst with several active sites, it is characterized by a wide molecular weight distribution of a polymer, and is limited in terms of securing of desired properties due to a non-uniform composition distribution of comonomers.
Meanwhile, the metallocene catalyst consists of a main catalyst, of which a main component is a transition metal compound, and a cocatalyst, which is an organometal compound including aluminum as the main component, and as such a catalyst is a homogeneous complex catalyst and a single site catalyst, it obtains a polymer with a narrow molecular weight distribution and a uniform comonomer composition distribution according to the properties of the single site catalyst, and can change the stereoregularity, copolymerization property, molecular weight, crystallinity, etc. according to modification of the ligand structure of the catalyst and change of polymerization conditions.
U.S. Pat. No. 5,032,562 describes a method for preparing a polymerization catalyst by supporting two different transition metal catalysts on one carrier. This is a method of producing a bimodal distribution polymer by supporting a titanium (Ti)-based Ziegler-Natta catalyst producing a high molecular weight and a zirconium (Zr)-based metallocene catalyst producing a low molecular weight on one carrier, and has disadvantages in that the supporting process is complicated, and polymer morphology is deteriorated due to the cocatalyst.
U.S. Pat. No. 5,525,678 describes a method of using a catalyst system for olefin polymerization that simultaneously supports a metallocene compound and a non-metallocene compound on a carrier, thus simultaneously polymerizing a high molecular weight polymer and a low molecular weight polymer. It has disadvantages in that the metallocene compound and the non-metallocene compound should be separately supported, and the carrier should be pretreated with many compounds for supporting.
U.S. Pat. No. 5,914,289 describes a method of controlling the molecular weight and molecular weight distribution of a polymer using metallocene catalysts supported on each carrier, but a large amount of solvent is required when preparing the supported catalyst and a long preparation time is required, and the used metallocene catalysts should be respectively supported on a carrier, which is inconvenient.
Korean Patent Application No. 2003-12308 discloses a method of controlling molecular weight distribution by supporting a binuclear metallocene catalyst and a mononuclear metallocene catalyst on a carrier together with an activator, and polymerizing while changing combinations of catalysts in the reactor. However, this method is limited in terms of simultaneous realization of the properties of each catalyst, and has disadvantages in that the metallocene catalyst part is isolated from the carrier component of the finished catalyst to induce fouling in the reactor.
Further, in the case of a copolymer of ethylene and an alpha-olefin polymerized using most transition metal compounds, a narrow molecular weight distribution is exhibited compared to a polyolefin obtained through the existing high pressure process, but in terms of the polymer structure, long chain branches are not included or a relatively small amount of long chain branches are included. Recently, there have been active attempts to obtain a polyolefin copolymer with a polymer structure having long chain branches and various properties in the academic and industrial worlds, but development of novel catalysts and processes for this are still required. Particularly, there is a continued demand for the development of catalysts with excellent performance that include long chain branches and thus can secure excellent processibility and mechanical properties, and simultaneously, have excellent transparency even when processed into a film.